The most common multibeam electron gun presently used in color picture tubes is the inline electron gun. An inline electron gun is one designed to generate or initiate preferably three electron beams in a common plane and direct those beams along convergent paths in that plane, to a point or small area of convergence at the tube screen.
Most inline electron guns attain static convergence of the undeflected electron beams by slightly distorting the focus fields at the outer beams, so that the outer beams are deflected toward the center beam to effect convergence of the beams at the screen. One means of distorting the focus fields is to offset one aperture in a focus electrode from its associated aperture in a facing focus electrode. A given static convergence at the screen of a tube is established by a particular combination of aperture offsets throughout the gun and beam position in the main lens. A problem, encountered in color picture tubes having built-in static convergence, is convergence drift during tube warm-up. Convergence drift is caused by a change of beam position in the main lens due to a relative change of horizontal aperture positions of all the electrodes throughout the electron gun. The relative aperture motion is caused by different thermal expansions of the different grids due to a temperature gradient from the cathode to the main lens.
The convergence drift problem has been approached previously by tailoring the coefficient of expansion of each electrode, to match the thermal gradient and keep the relative horizontal positions of all apertures throughout the gun constant. Such a modified electron gun is disclosed in U.S. Pat. No. 4,631,442, issued to Reule et al. on Dec. 23, 1986.
However, it was determined by the present inventors that simply matching the coefficients of expansion of the electrodes to the thermal gradient in an electron gun does not always provide the desired reduction in convergence drift.